Flex fuel: Should you reach for that nozzle?

Stop at nearly any gas station these days and there’s a good chance you’ll see it: a nozzle labeled “flex fuel.” It may cost less than regular gasoline, too.

What is this flex fuel, and do you want to run it? Judging by the comments we’ve seen online, those seemingly simple questions can cause all kinds of confusion.

What is flex fuel? It’s a blend of ethanol and gasoline often also labeled as E85.

So, what is E85? The E85 designation signifies the amount of ethanol present in the mix, so you might assume that E85 contains 85% ethanol and 15% gasoline. Sadly, you’d be a little off.

According to the ASTM International standards, E85 shall only contain 51% to 83% ethanol. The actual ratio of the two–plus everything else found in that fuel–can fluctuate depending on several factors, including the season and the retail location.

Does that ratio matter? As with most things in life, it depends. As explained by Zachary J. Santner, senior specialist of quality at Sunoco, a production car designed to use E85 can automatically accommodate either extreme–a mix higher in ethanol might deliver increased performance, while going the other way could improve efficiency.

Those tuning their car for max horsepower, however, will likely want to eliminate that variable by knowing exactly how much ethanol is present. Holley, Summit Racing and other motorsports suppliers sell simple E85 fuel testers for about $30. You fill a little tube with E85 and some water; how the two interact reveals the ethanol content. (Another option, Santner notes: Barrel E85 like Sunoco’s E85-R always contains exactly 85% ethanol.)

Can I use flex fuel in any vehicle? Most pump fuels contain up to 10% ethanol as it’s a relatively inexpensive way to increase octane, and modern fuel systems can usually tolerate the blend. Heavily oxygenated fuels like E85, though, require increased fuel delivery, meaning specialized fuel pumps and injectors. Unless a car has been converted to run on E85, only run flex fuel in vehicles configured to run it.

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